1. Field of the Invention
This invention relates generally to the detection of analytes, and, more particularly, to a system for detecting levels of analytes measured with a laser induced fluorescence sensor (LIFS).
2. Description of the Related Art
In a fiber optic based LIFS probe, an energy field propagates through a fiber to an active region. In the active region, evanescent or distal field excitation leads to a fluorescent signal from an analyte outside the fiber. The fluorescent signal propagates back through the fiber and is used to estimate the concentration of the analyte. Typical uses for such probes include detecting hazardous waste and monitoring the contamination of a process stream.
A conventional optical system for a probe, as described in Thompson et al., U.S. Pat. No. 5,141,312, issued Aug. 25, 1992, includes a laser which provides excitation light through an angled dichroic mirror and then through an objective into a fiber. Fluorescence is directed back through the probe and objective towards the mirror and reflected into a detection lens and filter for focusing the fluorescence at a detection point.
A complementary system is described by Block et al., U.S. Pat. No. 4,582,809, issued Apr. 15, 1986. In this design the excitation light is reflected by a dichroic mirror through an objective into a fiber, while returning fluorescence is passed first through the objective, and then through the mirror into a photodetector.
The use of an objective between the mirror and probe causes stray reflections and fluorescences in the detection path. In the system described in aforementioned U.S. Pat. No. 5,141,312, a concave perforated mirror passes the laser excitation and focuses the returning signal fluorescence onto a detector, and a reflecting microscope objective is used as the objective to reduce photoluminescence originating in the objective. This technique has the limitation that the objective is present in the detection path and may create stray reflections and fluorescences which are transmitted to the detector, particularly if the reflecting objective is replaced by a standard glass microscope objective. Another problem with conventional systems is their need for precise alignment and positioning of each of the elements.